TDP-43 is a 43 KD, ubiquitously expressed protein well conserved among the eukaryotes. This DNA/RNA-binding factor is predominantly located in the nucleus as a dimer, and it has been implicated to play roles in multiple cellular functions, e.g. transcriptional repression, splicing, and translation. TDP-43 has also been identified as the pathological signature protein of a range of neurodegenerative diseases. The pathological samples of these diseases, which have been termed TDP-43 proteinopathies, are characterized with cytoplasmic and, to a much less extent, nuclear TDP-43-positive (+) and ubiquitinated inclusions (UBIs) containing full-length TDP-43, poly-ubiquinated TDP-43, phosphorylated TDP-43, as well as 35 KD- and 25 KD carboxyl fragments of TDP-43. Of the two major categories of TDP-43 proteinopathies are frontotemporal lobar degeneration with ubiquitin (+) inclusions (FTLD-U) and amyotrophic lateral sclerosis (ALS). It has been estimated that approximately 50% of FTLD-U and 80-90% of ALS, which has an incidence rate between 1.5 and 2.5 per 100,000, are signified with TDP-43 (+) UBIs (6). Furthermore, numerous experimental data have suggested that mis-regulation of the metabolism of TDP-43, including the formation of the TDP-43 (+) UBIs, plays a causative role in the pathogenesis. Thus, it would be logical to identify the potential drugs of FTLD-U and ALS with TDP-43 proteinopathies, for which there is no effective drug therapy yet, by targeting either TDP-43 or the TDP-43 (+) inclusions.
Caccamo et al. (2009) overexpressed ˜25 kDa C-terminal fragment (the last 199 amino acids; C199-TDP) of TDP-43 in N2A mouse and SH-SYSY human neuroblastoma cell lines, leading to a redistribution/mislocalization of endogenous TDP-43 from nucleus to the cytoplasm. They reported that rapamycin treatment reduced the steady-state levels of ˜25 kDa fragment, decreased the mislocalization of TDP-43 to cytoplasm, but rapamycin had no effect on the amount of full-length TDP-43 in the C199-TDP transfected cells (JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 284, NO. 40, pp. 27416-27424). The C199-TDP transfected neuroblastoma cell line does not mimic the FTLD-U disease, nor displays symptoms of FTLD-U. It is not a scientifically recognized animal model for FTLD-U disease. Furthermore, patients with FTLD-U have type 3 ubiquitin-positive, TDP-43 positive, tau-negative pathology at post-mortem. Cytosolic TDP-43 (+) inclusions in patients with FTLD-U contain not only ˜25 kDa but also ubiquinated TDP-43, full-length TDP-43 and ˜35 kDa fragments. Clinical symptoms of FTLD are complex and progress with the stage of the disease. There are three clinical subtypes for FTLD: behavioural-variant frontotemporal dementia, semantic dementia and progressive nonfluent aphasia. The symptoms of frontotemporal dementia (FTD) can be classified roughly into two groups which underlie the functions of the frontal lobe: behavioural symptoms (and/or personality change) and symptoms related to problems with executive function. Semantic dementia (SD) is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains. The major symptom of progressive nonfluent aphasiais progressive difficulties with the production of speech. It is not known whether autophage enhancers would improve any clinical symptoms of patients with FTLD-U.
Moreover, there are contradicting reports about the relationship of autophage and cell death. Ravikumar et al. (2006) reported that rapamycin pretreatment reduced mitochondrial loads and protected against apoptosis including neuronal death (Human Molecular Genetics, Vol. 15, No. 7 1209-1216). However, Zhang et al. (2011) reported that treatment with autophage enhancer rapamycin caused more server mitochondria impairment, accelerated motor neuron degeneration, shortened the life span of the amyotrophic lateral sclerosis (ALS) mice, and had no obvious effects on the accumulation of in copper-zinc superoxide dismutase (SOD1) aggregates, suggesting that rapamycin treatment may exacerbate the pathological processing through apoptosis and other mechanisms in the ALS mice “Rapamycin treatment augments motor neuron degeneration in SOD1G93A mouse model of amyotrophic lateral sclerosis” Autophagy 7:4, 1-14).
A previously unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies, especially in connection with an effective drug for FTLD-U.